Serum secretin and gastrin levels are measured by radioimmunoassay in man and dogs. Fasting plasma secretin and gastrin levels and plasma secretin and gastrin responses to a meal and other agents are investigated in healthy subjects and patients with a variety of gastrointestinal disorders. Serum levels of both secretin and gastrin will be correlated with various biological actions produced by exogenous and endogenous secretin or gastrin. By measuring the levels of both hormones in the serum or plasma, the mechanisms of disordered physiology involved in digestive disorders will be investigated. BIBLIOGRAPHIC REFERENCES: Chey, W.Y., H.H. Tai, R. Rhodes, K.Y. Lee and J. Hendricks. Radioimmunoassay of Secretin: Further Studies. Thompson, J. (ed), Gastrointestinal Hormones, University of Texas Press, Austin, Texas, 1975, p. 269; Tai, H.H., B. Korsch and W.Y. Chey. Preparation of 125I-labeled Secretin of High Specific Radioactivity. Anal Bioch, in press, 1975.